1. Field of the Invention
This invention relates to drying granular or powdery material, preferably granular resin material, prior to processing thereof into intermediate or finished products, preferably by extrusion or molding.
2. Description of the Prior Art
Plastic resins are initially granular materials and are produced in pellets. These pellets are processed by extrusion or other means in which the granular resin pellets are heated until the pellets melt and may then be molded or extruded into a desired shape. Typically granular resins melt at elevated temperatures, for example from 300-400.degree. F., which is well above the boiling point of water.
Many granular resins have affinity for moisture. These hydroscopic resins absorb moisture and cannot be properly processed by molding or extrusion until dried. If processed before dry, moisture in the plastic boils at or approaching the high plastic molding or extrusion process temperatures, leaving bubbles and perhaps other imperfections in the finished product. Hence, hydroscopic granular resins must be dried prior to molding or extrusion.
Some granular resin materials are extremely hydroscopic and become unprocessable by molding or extrusion in ten minutes or less after exiting a dryer, due to the rapid absorption of moisture by the granular resin material.
It is known to dry granular resin material by placing the granular resin material pellets on large shallow trays to a depth of one or two inches, and putting those trays into ovens for several hours. With this approach to granular resin material drying, resin temperatures of 150-180.degree. F., but no higher, can be used since many granular resin materials begin to soften at 200-210.degree. F.
During the drying process, the granular resin material cannot be permitted to soften, since it becomes unmanageable. Once granular resin material begins to soften, at temperatures above the boiling point of water, the granular resin material pellets stick together in lumps or even melt into useless masses of solid plastic, making it impossible to further process the resin material into a useful article.